Entire fetal rat hearts transplanted to the anterior chamber of the eye proliferate and mature into adult hearts, maintaining the rhythmical activity associated with cardiac muscle. Ultrastructural and cytological examination of such transplants revealed the presence of cholinesterase positive fibers as well as formaldehyde-induced fluorescent nerve fibers in the transplant. Additionally, mylinated nerve fibers coursed throughout the transplants as well as nerve fibers which contained vesicular-like material presumed to be noradrenergic nerve fibers.